Unexpected side-effects of assigning to sys.modules[__name__]

[Steven D'Aprano]

Given this module:

#funny.py
import sys
print "Before:"
print "  __name__ =", __name__
print "  sys.modules[__name__] =", sys.modules[__name__]
sys.modules[__name__] = 123
print "After:"
print "  __name__ =", __name__
print "  sys =", sys


when I run it I get these results:


[steve at sylar python]$ python2.6 funny.py
Before:
  __name__ = __main__
  sys.modules[__name__] = <module '__main__' from 'funny.py'>
After:
  __name__ = None
  sys = None



I'm completely perplexed by this behaviour. sys.modules() seems to be a 
regular dict, at least according to type(), and yet assigning to an item 
of it seems to have unexpected, and rather weird, side-effects.

What am I missing?



-- 
Steven